Sounds like you're a little too war-happy. When you get to around 20-30 cities on a Standard sized map, slow or stop your conquest and shift gears toward building up a powerful production hub. You don't have to completely stop the war machine, but you want to avoid grinding the computer into the dust so much that its culture production stops.
I'm not the best player, and I don't go for speed, but I usually achieve a science victory on Immortal at around turn 300. If that's my plan from the outset, I concentrate on moves that will earn the most tech and civic boosts. I typically shift into end-game science mode as soon as I see the first AI-controlled Anti-Tank unit. At that point, I bee-line to Rocketry, then seek the space ship techs, picking up other techs when the prices drop enough (I can get it in two or three turns) or if I need one for some other reason. Most cities start running nothing but either projects or Builders at that point.
With Rise & Fall, you can significantly speed up a science victory by getting Reyna's end-point ability to buy districts. It takes a long time to build a Spaceport, but buying one is instantaneous. Also, get to the Tier 3 government of your choice to get access to the third building in the Government Plaza that lets you spend Builder charges on projects. You can use Builders to rush Space projects (1 per turn), which can launch your spaceship very quickly. With that in play, the space age Great Scientists and Engineers are pretty well unnecessary, although you may wish to grab them just to prevent the computer from doing so.
Without Rise & Fall, I usually re-locate all of my Trade Routes to my production hub and send them out to whatever city will get me the most cogs. Getting Rurh Valley in a high-production city helps a lot, and those Great People are much more important if you don't have the benefit of the Government Plaza.
